Implement fallback search logic when ICU is unavailable

[b6k-dev]

Closes https://github.com/microsoft/edit/issues/172

Demo:

https://github.com/user-attachments/assets/56c633f3-51ac-41be-a3f0-e66a3d29489c

This pr introduces a fallback Boyer–Moore–Horspool algorithm used for searching the buffer when the ICU library fails to load.

The implementation is working, however since I'm still getting used to writing rust and the time I can devote is quite limited the code probably needs some polishing (maybe better structuring is needed to reuse as much as possible between the ICU and non-ICU search operations). Anyways I guess it is a good foundation to iterate on.

[lhecker]

The implementation is working, however since I'm still getting used to writing rust

Meanwhile you delivered two of the most outstanding PRs this project has received so far. Don't get impostor syndrome! 😅

FYI: I'll try to first review and merge #255 before merging this PR, unless you'd like to prioritize this PR.